Fix for SIP endpoints starting with "sip"

Currently registrations don't happen if the endpoint username starts with the letters 'sip'
For eg. 
sipadmin@jssip.com

The ERROR being, 

    MESSAGE | Error parsing "To" header field with value: "sipadmin@jssip.com;tag=b6a60d2210e07fdf58fd2b188ac28a63.c13e"